The present invention relates generally to a process for adjusting the supply pressure within a fluid pumping system, for example, fluid flowing through a complex distribution grid, by varying the speed of the pump supplying the fluid to the system. By adjusting the pressure generated by the pump in response to fluctuating demand rather than maintaining a constant pump pressure, utility costs can be reduced and equipment life may be increased.
Output demand within a given fluid distribution system may vary at any given time due to numerous factors. In public water distribution systems, for example, these factors may include the time of day, the day of the week, and weather conditions. Pump output within the fluid distribution system must generally be sufficient to maintain a predetermined pressure. However, as water usage increases, the pressure in the fluid distribution grid generally decreases; conversely, as the water usage decreases, the pressure in the distribution grid generally increases.
Because it is generally desirous to maintain some predetermined pressure within the fluid distribution system, pump output is typically set to maintain this predetermined pressure at maximum demand. Thus, there is typically a constant state of high pressure in these fluid distribution systems, especially during off-peak hours. Maintaining this high-pressure state requires a considerable expenditure of energy as well as contributing to fluid leakage and wear in the distribution grid.
Conventional control systems that attempt to maintain a constant output pressure at their discharge often fail to maintain the same pressure inside a typically complex piping grid, largely due to nonlinear losses caused mostly by friction. Most older, conventional control systems often employ only control valves, or alternatively, may simply repeatedly start and stop the supply pump to adjust pressure. Control valves are relatively ineffective to control pressure in a large distribution system, and frequent starting and stopping subjects a pump""s motor to a high-in rush current and can cause water hammer problems. Such results may lead to higher energy costs and reduced equipment life.
More modern systems may make use of variable frequency drives (VFD""s) to adjust pressure by varying pump output (speed). In order to compensate for pressure fluctuations within the fluid distribution system, these VFD control systems may employ pressure sensors to send signals to a programmable logic controller (PLC), which may then vary the pump speed in response thereto. However, even with known VFD control systems, generally only a small number of fixed set points are used to vary pump output, such as, for example, a set point for each of day time and night time operation. Therefore, what is needed, but has been heretofore unavailable, is a control system that utilizes multiple set points to continuously vary the target pressure in response to changing demands on the system.
The system and method of the present invention satisfies this need. The present invention discloses an interactive process that monitors and continuously adjusts the fluid pressure within a fluid distribution system so that adequate distributed pressure is maintained throughout. The present system generates and uses derived functions that are components of the process, to alter a reference set point and create a new, effective set point. The speed of the pump motor can then be continuously adjusted to maintain the system at a pressure closer to the effective set point. The creation of a continually variable effective set point as contemplated by the present invention allows for more accurate system pressures, and also permits reduced pump output during periods of low inflow. Therefore, the present invention allows for reduced stress on the fluid distribution system components, which in turn may reduce energy costs, fluid leakage, and equipment aging.
In addition to the novel features and advantages mentioned above, other objects and advantages of the present invention will be readily apparent from the following descriptions of the drawings and exemplary embodiments.